Determining the location of nodes, in a wireless communication network (e.g., an ad-hoc communication network), can facilitate the association of the information transmitted by each node, to the spatial location of each node either in the network or in a geographical coordinate system or in both. For example, in an emergency situation, the location of rescue teams, ambulances and fire trucks, may provide valuable information regarding the progression of the rescue operations. In a military environment, the location of troops or vehicles helps the commander to determine the best course of action. In cellular networks, the location of a node may be of interest to other users (e.g., friends of the person possessing the phone or tracking a stolen phone).
A known in the art method, for determining the geographical location of a node in a wireless network, includes coupling each node with a Global Positioning System (GPS) and have the node transmit the geographical location determined by the GPS. A known in the art method, for determining the relative location of a node in the network, includes determining the Time Of Arrival (TOA) of a transmitted signal from another node. According to this method, a first node transmits a first signal to a second node and records the time of transmission. The second node receives this first signal and records the time of reception. Thus, the transmission propagation delay between the nodes can be determined according to the recorded times of transmission and reception. Multiplying the determined transmission propagation delay by the propagation speed yields the relative distance between the nodes. When the relative distance between a plurality of network nodes is known, the relative location of the nodes in the network can be determined, for example, by using trilateration.
Reference is now made to FIG. 1, which is a schematic illustration of the transmission of signals according to the TOA method which is known in the art. Line 10 represents time. At time T1, node A transmits a signal to node B and records time T1. Node B receives the signal at time T2, and records time T2. When the clocks of node A and node B are synchronized (i.e., the clocks do not exhibit a relative clock-shift and clock-drift therebetween), the propagation time, TPAB, of a signal between node A and node B is determined according to:TPAB=T2−T1  (1)Multiplying the propagation time TPAB by the propagation speed C, results in the distance, RAB, between node A and node B.RAB=TPAB*C  (2)Thus, by determining the distances to several adjacent nodes, each node in the network can determine the location thereof, relative to the other nodes in the network (e.g., by employing triangulation).
PCT publication WO 2005/081012, to Cheok et al, entitled “Ultra Wide Band Navigation System With Mobile Base Stations”, directs to a method for determining the location of a target by using closed-from triangulation with three of more mobile base stations. According to the method directed to by Cheok, four base stations are used, and the location of these base stations, relative to each other, is determined. A target determines the location thereof, relative to these base stations, according to signals received form the four base stations. In one embodiment directed to by Cheok, the base stations and the target determine the relative locations thereof according to the TOA method. According to another embodiment directed to by Cheok, the base stations and the target determine the relative locations thereof according to the Time Difference Of Arrival method (TDOA).
According to the TDOA, a designated base station transmits an initial signal at time T0. When the other base stations receive this signal, the base stations (i.e., including the designated base station) wait for a predetermined time period TDi (i.e., the time delay is different for each base station designated by i). Thereafter, each base station transmits a signal associated therewith. The target unit receives the signal from the base station and clocks the time the signals are received. The time difference between the time the target unit received each signal from each base station, and the time the designated station transmitted the initial signal is related to the distance between the target unit and the base station transmitting the signal. However, the geometrical relationships, between the target unit and the base stations, are non-linear. According to the method directed to by Cheok et al, the range differences, between the target unit and the designated base station, and between the target unit and each one of the other base station, is used to reduce the non-linear relationships to linear relationships.
U.S. application publication US 2004/0005902 to Belcea, entitled “System and Method for Correcting the Clock Drift and Maintaining the Synchronization of Low Quality Clocks in Wireless Networks”, directs to a method for calculating the clocks shift, clock drift and propagation delay values using series of message exchanges. According to the method directed to by Balcea, A node, A, transmits a message to another node, B, at time tA1 (i.e., according to the clock of node A) corresponding to time tB1 at node B, tB1 is equal to tA1 plus the time shift between the clocks of node A and node B, ΔAB. At time tB2, node B receives the message transmitted by node A. At time tB3, node B responds to the message sent by node A with a first message containing the value of the clock, at node B, when the message from node A was received (i.e., tB2). The message from node B is received at node A at tA4. Shortly after node B transmits the first message, node B transmits a second message containing the clock value, at node B, when the first message, form node B to node A, was transmitted (i.e., tB3). Node A now has information relating to tA1, tB2, tB3 and tA4. Node A determines the values of the clock shift and the propagation delay between node A and node B according to:tA4−tB3=−ΔBA+pBA  (3)tB2−tA1=ΔBA+pAB  (4)Where ΔBA is the clock shift between node B and node A and pAB is the propagation delay between node A and node B. It is assumed that the propagation between node B to node A, pBA, is also pAB. Thus, solving two equations (i.e., equations (3) and (4)) with two unknowns (i.e., ΔBA and pAB) yields the clock shift and the propagation delay between node A and node B. By repeating the process every predetermined time period, the clock drift (i.e., the rate of change of the clock shift) between node A and node B is determined.